Studying the evolution of the pore structure of coal during spontaneous combustion is of great value in further understanding the mechanism of coal spontaneous combustion (CSC) and its prevention. In this study, we selected three low-rank coals and used nuclear magnetic resonance (NMR) to visualize the macroscopic evolution of the pore structure of coal after heat treatment and to analyze the effect of temperature (25-500 °C) on the pore structure of coal, including porosity, permeability, and fractal dimensions. The obtained results show that the overall NMR signal in coal increases with increasing temperature, indicating that heat treatment can induce the enlargement, opening, and interconnection of pores and fractures in coal. The equivalent average pore radius (rm) of coal shows a positive correlation with temperature, with a substantial increase in rm, especially after temperatures above 200 °C. During heating, the porosity and permeability of all three coals tended to increase with temperature. At temperatures above 300 °C, the permeability of coal dramatically increases, predicting a higher fluid transport capacity. Furthermore, NMR multifractal theory was proposed for quantitative pore space dimensional characterization. The obtained results show that the fractal dimensions of the adsorption space of coal pores increase and then decrease with temperature during heating, while the fractal dimensions of percolation space are negatively correlated with temperature. In addition, the dimensions of adsorption space vary more strongly than those of percolation space, meaning that the adsorption capacity of low-rank coals is more significantly influenced by temperature.

Due to the COVID-19 pandemic, human activities are largely restricted in Shanghai, China and it is a valuable experiment to testify the correlation of air quality and human activities. In consideration of the complexity of air pollution, this study aims to compare the multifractal characteristics of air quality index (AQI) time series before and during COVID-19 partial lockdown, and analyze the correlations between multifractal parameters of AQI time series and human activities in Shanghai, China. The hourly AQI series in Shanghai from November 27, 2019 to March 23, 2020 is used for this study. Firstly, using the MF-DFA method, the multifractal characteristics of the AQI series are explored. Secondly, the causes of the multifractality of the AQI series are determined. Finally, the correlations between multifractal parameters of AQI time series and human activities are investigated. The multifractal analysis results reveal that the AQI series during COVID-19 partial lockdown also has multifractal characteristics, and the slightly weaker multifractal characteristics and marginally smaller multifractal degree are obtained in comparison with the pre-lockdown phase. However, the contribution of the effective or intrinsic multifractality before and during COVID-19 partial lockdown are very close. The results via the sliding window procedure indicate that the multifractal parameters ( Δ H , Δ α , Δ f ) show the similar fluctuations along with the fluctuations of passenger volume in Shanghai Metro. Furthermore, it is found that Δ H , Δ α and Δ f and adjusted passenger volume in Shanghai Metro are positively correlated. The possible trend is that the higher adjusted passenger volume is, the larger the value of Δ H , Δ α , Δ f becomes, which means the stronger multifractal characteristics and larger multifractal degree of air quality system.

This paper explores the multifractal features of different commercial designations of Iberian ham (acorn 100% Iberian ham, acorn Iberian ham, feed/pasture Iberian ham and feed Iberian ham). This study has been done by taking as input the fatty infiltration patterns obtained from digital image analysis of ham cuts comparing mechanic and manual slicing. The yielded results show the multifractal nature of fatty connective tissue in Iberian ham, only when knife cutting is applied, confirming the differences between the designations according to their genetics and feeding. Thus, the multifractal parameters presented in this work could be considered as additional information for checking Iberian ham quality by using non-destructive methods based on the combination of image analysis and predictive techniques. Meat industry can take advantage of these methods to evaluate meat products, especially when fat-connective tissue with complex pattern distribution is involved.